This invention relates generally to measuring apparatus for very light masses, and more particularly to an electronic balance capable of correcting an error in the measured value due to off-center placement of the mass to be weighed.
There are many ways to measure weight. One problem they all face is that of off-center pan loading in which the mass to be weighted is not located at the exact center of the weighing pan, but is off to one side. In general, when a load is placed deviatedly from a required point of placement, such as the the center of the pan, the bending moment exerted on the pan thereby produces an error in the measured value. In the current state of the art there are two basic ways of handling the problem of off-center loading. The present invention discloses a third way.
The two basic ways of handling the problem of off-center pan loading in the current state of the art consist of: "hanging pan" and "parallel-motion" techniques. The "hanging pan" technique consists of a pan suspended from a knife-edge support at a single point, such that gravity will twist the pan so as to locate its center of gravity directly below the pan support. This prevents bending moments in the load sensing mechanism. With the "parallel-motion" technique a parallel-motion linkage is attached to the pan. This absorbs the bending moments caused by off-center loading and prevents their being applied to the loadcell.
The hanging pan technique is mechanically simple with a simple sensor. It has a very tall vertical scale dimension, but no adjustments for off-center loading are required. However, friction at the knife-edge and overall inertia are sources of error when weighing very light masses. Devices constructed in accordance with this technique tend to be bulky, and the pan's hanger is often in the way when loading. Leveling of the overall device is important.
Devices constructed in accordance with the parallel-motion technique are mechanically complex. They are top-loading and have a thick, but simple sensor. The vertical scale dimension is less than that of a hanging pan, but is still substantial. Adjustment for off-center loading is required through mechanical adjustments of linkage. Misadjustment of the linkage is a common source of error. Leveling of the device is very important.
U.S. Pat. No. 4,401,173 and 4,482,022 to A. Komoto disclose an electronic balance constructed in accordance with the parallel-motion technique, but with the addition of multiple sensors for detecting a possible deviation of the load acting point from the required point on the pan, and with the addition of electronics for correcting an error in the measured value due to the said deviation. The key to Komoto is the detection and measurement of a possible deflection occurring in the pan support (12) due to the turning moment exerted on the pan by a load being placed deviatedly from a required point of placement, such as the center of the pan. According to Komoto, the deflection occurring in the pan support (12) will be in the horizontal X-Y plane. Komoto also discloses a version for use in large-scaled electronic balances in which a pan is supported by two supports which in turn provide force acting lines to two levers each resting on a fulcrum. The levers are connected to each other by means of a pin and are connected to a load measuring section.